Following up on his use of twins as proof against astrology, Augustine turns to the Biblical example of Jacob and Esau. They were born twins, but were all intents and purposes complete opposites both in character and in the events of their lives.
The astrologers (and physicians who rely on astrology) themselves are guilty of rampant inconsistency both in the matter of twins and in the matter of children who are born at the same moment (and hence under the same stars). "What strange circumstance is this, that two children conceived at the same time, nay, at the same moment, under the same position of the stars, have different fates which bring them to different hours of birth, whilst two children, born of two different mothers, at the same moment of time, under one and the same position of the stars, cannot have different fates which shall conduct them by necessity to diverse manners of life and of death?"
And of course we have the example of fraternal twins of different genders. To be sure, Augustine didn't know the biology at work in conception in quite the same detail that we do, but it doesn't affect the fact that you have a boy and a girl born under the same sign and obviously with different destinies ahead of them.
So far Augustine has not even needed to draw on Scripture. Reason alone has been enough to show the silliness and falsehood of astrology. As he repeatedly point out, from our perspective that which drives the events of the world is the will, which is not under the power of the stars. (He will of course later point out that even the will is not in control but is under the control of Providence.) This is a good model for us: while Scripture is the only source of truth for the Christian life, we are allowed and encouraged to use all the tools at our disposal to guide the world in separating truth from error.